[llvm] [SCEV] Handle backedge-count logic for std::reverse like loops (PR #92560)

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hiraditya wrote:

LGTM as i dont have any pending concerns. I'd defer to code owners to +1. Thanks for working on it as this enables vectorization of a class of loops. One suggestion would be to run llvm-testsuite or SPEC and see how many new loops got vectorized. Even without getting performance numbers, just `-Rpass=vectorize -Rpass-analysis=vectorize` stats will be useful.

https://github.com/llvm/llvm-project/pull/92560